Phospholipase D signaling: orchestration by PIP2 and small GTPases

Abstract

Hydrolysis of phosphatidylcholine by phospholipase D (PLD) leads to the generation of the versatile lipid second messenger, phosphatidic acid (PA), which is involved in fundamental cellular processes, including membrane trafficking, actin cytoskeleton remodeling, cell proliferation and cell survival. PLD activity can be dramatically stimulated by a large number of cell surface receptors and is elaborately regulated by intracellular factors, including protein kinase C isoforms, small GTPases of the ARF, Rho and Ras families and, particularly, by the phosphoinositide, phosphatidylinositol 4,5-bisphosphate (PIP(2)). PIP(2) is well known as substrate for the generation of second messengers by phospholipase C, but is now also understood to recruit and/or activate a variety of actin regulatory proteins, ion channels and other signaling proteins, including PLD, by direct interaction. The synthesis of PIP(2) by phosphoinositide 5-kinase (PIP5K) isoforms is tightly regulated by small GTPases and, interestingly, by PA as well, and the concerted formation of PIP(2) and PA has been shown to mediate receptor-regulated cellular events. This review highlights the regulation of PLD by membrane receptors, and describes how the close encounter of PLD and PIP5K isoforms with small GTPases permits the execution of specific cellular functions.

Fig. 1

Regulation and cellular roles of PLD and PIP5K. Regulation of PLD and PIP5K by ARF and Rho family GTPases is essentially involved in the regulation of intracellular vesicle trafficking and actin cytoskeleton reorganization. Both PLD and PIP5K are stimulated by cell surface receptors and by conventional PKC isoforms, and the latter can become activated after receptor-induced hydrolysis of PIP₂ by PLC. Positive feed-forward regulation is achieved by stimulation of PLD by PIP5K-derived PIP₂, and of PIP5K by PLD-derived PA. Activation of ARF-GAPs by PIP₂ accelerates the inactivation of ARF proteins, and may terminate a round of PA and PIP₂ synthesis

Fig. 2

Regulation of PLD by the M₃ muscarinic receptor and receptor tyrosine kinases in HEK-293 cells. In human embryonic kidney (HEK-293) cells, signaling to PLD by the M₃ muscarinic receptor and by typical RTKs (EGF, PDGF, insulin) is organized into rather discrete pathways and channeled by particular heterotrimeric G proteins and small GTPases (orange), specific GEF proteins (pink) and further signaling components (green). AC, adenylyl cyclase; ROCK, Rho-kinase